Incorrectly set output cell name if using DRC::cell

[klayoutmatthias]

For details see https://www.klayout.de/forum/discussion/2370

This small DRC script throws an error

cell("TOP")
pi1 = input(1, 0)
pi1.output(2, 0)

workaround is to specify an output cell:

cell("TOP")
output_cell("TOP")
pi1 = input(1, 0)
pi1.output(2, 0)

Root cause is this line in _drc_engine.rb, method 'output_cell' which should use the cell object rather than the cell name for the cell argument of LayoutOutputChannel:

            @def_output = LayoutOutputChannel::new(self, @def_layout, cellname.to_s, nil)